There has been well known a photographic process to obtain a direct positive image without conducting a reversal process or without using a negative film.
As a process of forming a positive image using a conventionally known direct positive silver halide photographic light-sensitive material, there is known a process of using a non-pre-fogged internal latent image type silver halide photographic emulsion. This process comprises imagewise exposing the light-sensitive material, then subjecting the material to fogging treatment, and simultaneously or thereafter subjecting the material to surface development.
The term "non-pre-fogged internal latent image type silver halide photographic emulsion" means a silver halide photographic emulsion in which the grains have a core/shell structure and have sensitivity specks substantially inside of the silver halide grains so that a latent image is formed mainly inside of the grains when the grains are exposed to light.
Various processes for obtaining a direct positive image are described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577; and U.K. Patents No. 1,151,363, No. 1,150,553 and No. 1,011,062.
The mechanism of the direct positive image formation is considered as follow. Upon the imagewise exposure, so-called "internal latent image" is formed inside of silver halide grains. Then, by the subsequent fogging treatment, a surface desensitizing action caused by the internal latent image takes place. Through the surface desensitizing action, development specks (fog specks) are selectively formed only on the surface of unexposed silver halide grains, without forming development specks on the surface of exposed silver halide grains. Then, a photographic image (positive image) is formed within the unexposed area by the normal surface development process. The process of the above-mentioned fogging treatment can be performed by the so-called "light fogging method", in which whole surface of the light-sensitive layer is exposed to light, or the so-called "chemical fogging method", in which a nucleating agent is used.
A direct positive silver halide photographic light-sensitive material using such non-pre-fogged internal latent image type silver halide emulsion as mentioned above has been recently used for preparing a color reversal paper, a color hard copy or a color proof, because the material can be easily processed.
In the case of practically using the direct positive photographic light-sensitive material for the above-stated purpose, it is desired that an image provided by the light-sensitive material has a high maximum density and a low minimum density and also has excellent whiteness. The term "whiteness" means a color reproducibility when a white object is photographed. Since the above-mentioned light-sensitive material is restricted in the exposure region, the minimum density of an image provided thereby is an important factor. In addition, it is very important to make gradation of a toe part (in a characteristic curve) of the image much harder for the purpose of obtaining an excellent reproducibility, because the gradation of the toe part has an influence on the whiteness. (The whiteness is more increased as the gradation of a toe part of an image is made harder.)
There is known a method of incorporating a metal such as Mn, Pb, Zn or Cd into the non-pre-fogged internal latent image type silver halide grains to obtain an image having a high maximum density, a low minimum density and a hard gradation of the toe part, as described in Japanese Patent Provisional Publication No. 1(1989)-145647.
On the other hand, the light-sensitive material is desired to have a high sensitivity. To make the sensitivity of the light-sensitive material higher, an emulsion used therefor is generally subjected to chemical sensitization and spectral sensitization in the preparation stage of the emulsion. The spectral sensitization is carried out by adding a spectral sensitizing dye to the photographic emulsion as described in the above publication. In general, after the preparation of the emulsion (after the chemical sensitization of the emulsion) or during the preparation of a coating solution containing the emulsion (a coating solution for forming a light-sensitive layer), a spectral sensitizing dye is used. That is, the spectral sensitizing dye is introjuced prior to the coating process.
In the case of storing a direct positive photographic light-sensitive material for a long period of time (especially in the case of storing at a high temperature such as a temperature of 50.degree. C. and 55% RH for 10 days), the following problems are observed after the storage: the light-sensitive material varies in the sensitivity, and an image provided by the material has a soft gradation in the toe part and has a high minimum density to reduce the whiteness. To solve these problems, the aforementioned method of incorporating a metal into the silver halide grains is effective, but further improvements are desired.